Study on public exposure risk assessment for dismantling of radioactive components in decommissioning phase of nuclear reactor facilities

原子炉施設の廃止措置段階の放射性機器の解体における公衆被ばくリスクに関する研究

島田 太郎 ; 椛田 和彦*; 高井 静霞 ; 武田 聖司 

Shimada, Taro; Kabata, Kazuhiko*; Takai, Shizuka; Takeda, Seiji

原子力発電所の廃止措置段階における原子力規制検査は、リスク情報に基づいて実施される必要があるが、このリスクを定量的に評価する手法は開発されていない。そこで本研究では、廃止措置段階で発生する可能性のある事故事象のイベントツリーを作成し、解体対象機器ごとに事故シーケンスに応じた被ばく線量と発生確率の積で表される被ばくリスクを評価するコードDecAssess-Rを開発した。特に、事故時にHEPAフィルターに蓄積し、一気に放出される可能性のある移動放射能量は、解体作業の進捗に伴い時間的・空間的に変化することを考慮した。イベントツリーは、廃止措置段階及び類似の解体・交換作業における国内外のトラブル情報の調査結果をもとに構築した。事象頻度は一般産業界の情報に基づいており、事象進展確率は運転段階における機器故障確率に基づいている。また、廃止措置の進捗に伴い低減する安全機能については、解体作業のスケジュールに基づき考慮した。我が国におけるBWR及びPWRの解体作業の被ばくリスク評価の結果、火災事象の被ばくリスクが最も大きかった。特に、原子炉建屋内機器の気中切断による解体作業では、水中解体作業で最も放射能が大きい炉内構造物の解体作業よりも被ばくリスクが大きかった。

Nuclear regulatory inspections during the decommissioning phase of nuclear power plants need to be conducted based on risk information, but a method for quantitatively evaluating this risk has not been developed. Therefore, in this study, an event tree of accident events that may occur in the decommissioning phase has been developed, and a code DecAssess-R has been developed to evaluate the exposure risk, which is expressed as the product of the exposure dose and probability of occurrence according to the accident sequence for each equipment to be dismantled. In particular, we have taken into account that the amount of mobile radioactivity that may accumulate in HEPA filters and be released all at once during an accident varies temporally and spatially with the progress of dismantling work. The event tree was constructed based on the results of the survey of domestic and international trouble information in the decommissioning phase and similar dismantling and replacement operations. The event frequencies are based on information from general industries, and the event progression probabilities are based on the equipment failure probabilities in the operation phase. The safety functions to be reduced with the progress of decommissioning were taken into account according to the dismantling work schedule. As a result of the exposure risk assessment for dismantling operations of BWRs and PWRs in Japan, the exposure risk for fire events was the largest. In particular, the exposure risk was greater for the dismantling of components in the reactor building by airborne cutting than for the dismantling of reactor internals, which has the greatest radioactivity in underwater dismantling.